Abiraterone is a potent and selective inhibitor of CYP17 (17α-hydroxylase/C17,20-lyase). As Abiraterone was poorly bioavailable and also susceptible to hydrolysis by esterases, a prodrug was developed. Abiraterone acetate (A) was found to be resistant to esterases and was rapidly deacetylated to Abiraterone (B) in vivo, resulting in potent CYP17 inhibition. Abiraterone acetate is designated chemically as (3β)-17-(3-pyridinyl) androsta-5,16-dien-3-yl acetate and its structure is:

Abiraterone acetate is a white to off-white, non-hygroscopic, crystalline powder. Its molecular formula is C26H33NO2 and it has a molecular weight of 391.55. Abiraterone acetate is a lipophilic compound with an octanol-water partition coefficient of 5.12 (Log P) and is practically insoluble in water. The pKa of the aromatic nitrogen is 5.19.
Inactive ingredients in the Zytiga® tablets are colloidal silicon dioxide, croscarmellose sodium, lactose monohydrate, magnesium stearate, microcrystalline cellulose, povidone, and sodium lauryl sulfate.). Each Zytiga® tablet contains 250 mg of Abiraterone acetate.
Abiraterone acetate (ZYTIGA) is converted in vivo to Abiraterone, an androgen biosynthesis inhibitor, that inhibits 17α-hydroxylase/C17,20-lyase (CYP17). This enzyme is expressed in testicular, adrenal, and prostatic tumor tissues and is required for androgen biosynthesis.
CYP17 catalyzes two sequential reactions: 1) the conversion of pregnenolone and progesterone to their 17α-hydroxy derivatives by 17α-hydroxylase activity and 2) the subsequent formation of dehydroepiandrosterone (DHEA) and androstenedione, respectively, by C17,20 lyase activity. DHEA and androstenedione are androgens and are precursors of testosterone. Inhibition of CYP17 by Abiraterone can also result in increased mineralocorticoid production by the adrenals.
Androgen sensitive prostatic carcinoma responds to treatment that decreases androgen levels. Androgen deprivation therapies, such as treatment with GnRH agonists or orchiectomy, decrease androgen production in the testes but do not affect androgen production by the adrenals or in the tumor.
Abiraterone acetate decreased serum testosterone and other androgens in patients in the placebo-controlled phase 3 clinical trial. It is not necessary to monitor the effect of Abiraterone on serum testosterone levels.
Changes in serum prostate specific antigen (PSA) levels may be observed but have not been shown to correlate with clinical benefit in individual patients.
Following administration of Abiraterone acetate, the pharmacokinetics of Abiraterone and Abiraterone acetate have been studied in healthy subjects and in patients with metastatic castration-resistant prostate cancer (CRPC). In vivo, Abiraterone acetate is converted to Abiraterone. In clinical studies, Abiraterone acetate plasma concentrations were below detectable levels (<0.2 ng/mL) in >99% of the analyzed samples.
Following oral administration of Abiraterone acetate to patients with metastatic CRPC, the median time to reach maximum plasma Abiraterone concentrations is 2 hours. Abiraterone accumulation is observed at steady-state, with a 2-fold higher exposure (steady-state AUC) compared to a single 1,000 mg dose of Abiraterone acetate.
At the dose of 1,000 mg daily in patients with metastatic CRPC, steady-state values (mean±SD) of Cmax were 226±178 ng/mL and of AUC were 993±639 ng*hr/mL. No major deviation from dose proportionality was observed in the dose range of 250 mg to 1,000 mg. However, the exposure was not significantly increased when the dose was doubled from 1,000 to 2,000 mg (8% increase in the mean AUC).
Systemic exposure of Abiraterone is increased when Abiraterone acetate is administered with food. Abiraterone Cmax and AUC0-∞ were approximately 7- and 5-fold higher, respectively, when Abiraterone acetate was administered with a low-fat meal (7% fat, 300 calories) and approximately 17- and 10-fold higher, respectively, when Abiraterone acetate was administered with a high-fat (57% fat, 825 calories) meal. Given the normal variation in the content and composition of meals, taking Zytiga® with meals has the potential to result in increased and highly variable exposures. Therefore, no food should be consumed for at least two hours before the dose of Zytiga® is taken and for at least one hour after the dose of Zytiga® is taken. The tablets should be swallowed whole with water.
Abiraterone is highly bound (>99%) to the human plasma proteins, albumin and alpha-1 acid glycoprotein. The apparent steady-state volume of distribution (mean±SD) is 19,669±13,358 L. In vitro studies show that at clinically relevant concentrations, Abiraterone acetate and Abiraterone are not substrates of P-glycoprotein (P-gp) and that Abiraterone acetate is an inhibitor of P-gp. No studies have been conducted with other transporter proteins.
Following oral administration of 14C-abiraterone acetate as capsules, Abiraterone acetate is hydrolyzed to Abiraterone (active metabolite). The conversion is likely through esterase activity (the esterases have not been identified) and is not CYP mediated. The two main circulating metabolites of Abiraterone in human plasma are Abiraterone sulphate (inactive) and N-oxide Abiraterone sulphate (inactive), which account for about 43% of exposure each. CYP3A4 and SULT2A1 are the enzymes involved in the formation of N-oxide Abiraterone sulphate and SULT2A1 is involved in the formation of Abiraterone sulphate.
In patients with metastatic CRPC, the mean terminal half-life of Abiraterone in plasma (mean±SD) is 12±5 hours. Following oral administration of 14C-abiraterone acetate, approximately 88% of the radioactive dose is recovered in feces and approximately 5% in urine. The major compounds present in feces are unchanged Abiraterone acetate and Abiraterone (approximately 55% and 22% of the administered dose, respectively).
The usual dose is 4 tablets (1,000 mg) taken together once a day. The tablets have to be swallowed with a glass of water on an empty stomach. The tablets have to be taken at least one hour before food, or at least 2 hours afterwards. Abiraterone has to be taken with a steroid called prednisolone to help reduce some of the side effects.
In clinical studies following the oral administration of Abiraterone acetate Abiraterone exhibited variable pharmacokinetics and an exceptionally large positive food effect. Abiraterone Cmax and AUC0-∞ (exposure) were increased up to 17- and 10-fold higher, respectively, when a single dose of Abiraterone acetate was administered. In order to control Abiraterone plasma concentrations Zytiga® must be taken on an empty stomach. No food should be consumed for at least two hours before the dose of Zytiga® is taken and for at least one hour after the dose of Zytiga® is taken. The administered dose is also very large with 1 g taken once daily. Improving the oral bioavailability of the compound in the fasted state would therefore deliver two advantages: the abandoning of the requirement of taking the drug on an empty stomach and significant dose reduction. Based on the extent of the food effect of the currently used formula total elimination of it would allow 10-fold reduction of the dose.
In order to overcome the problems associated with prior conventional Abiraterone acetate formulations and available drug delivery systems novel complex formula of Abiraterone acetate and complexing agents and pharmaceutically acceptable excipients characterized by increased apparent solubility, instantaneous dissolution, reduced food effect which allows significant dose reduction and the abandoning of the requirement of taking the drug on an empty stomach.
A variety of strategies have been used to attempt to overcome these issues, see for example CN101768199A, CN102558275A, WO2014083512A1, WO2014145813A1, CN102321142A, WO2014102833A2, WO2014009436A1, WO2014145813A1, WO2014009434A1, WO2009009132A1, WO2013164473A1, WO1995011914A1, CA2513746A1, WO2010078300A1, WO2014100418A2 and WO2014009437A1.